Fuel cells for combining hydrogen and oxygen to produce electricity are well known. A known class of fuel cells includes a solid-oxide electrolyte layer through which oxygen anions migrate to combine with hydrogen atoms to produce electricity and water; such fuel cells are referred to in the art as “solid-oxide” fuel cells (SOFCs).
In some applications, for example, as an auxiliary power unit (APU) for an automotive vehicle, an SOFC is preferably fueled by “reformate” gas, which is the effluent from a catalytic hydrocarbon oxidizing reformer. Reformate typically includes amounts of carbon monoxide (CO) as fuel in addition to molecular hydrogen. The reforming operation and the fuel cell operation may be considered as first and second oxidative steps, respectively, of the liquid hydrocarbon, resulting ultimately in water and carbon dioxide. Generally, both reactions are exothermic, and both are preferably carried out at relatively high temperatures, for example, in the range of 700° C. to 1000° C. In cases where anode gas is recycled to the reformer, the reformer reaction may be endothermic in nature. A complete fuel cell stack assembly includes a plurality of components known in the art as interconnects, which electrically connect the individual fuel cells in the stack, as well as air supply and cooling means.
It is known in the art to recycle a portion of the tail gas from the stack anodes into the inlet to the reformer, which improves stack power density and system efficiency and reduces carbon precipitation and deposition in the system. Such recycling is known to be carried out by aspirating tail gas via a high temperature ejector or venturi. A shortcoming of this approach is that it requires a high-pressure flow source in the system, which may not always be available, and further, control may become unstable under low-flow conditions.
What is needed in the art is a means for reliably providing a controlled flow of SOFC stack anode tail gas into a reformer supplying reformate to the SOFC.
It is a principal object of the present invention to increase reliability of recycling of anode tail gas in an SOFC system.